1. Field of the Invention
The present invention relates to a nonaqueous secondary battery. More particularly, the present invention relates a nonaqueous secondary battery, such as a lithium-ion secondary battery, having excellent battery characteristics and being improved in safety.
2. Description of the Related Art
A nonaqueous secondary battery in which a metal oxide is used as a positive electrode, an organic electrolytic solution is used as an electrolyte, a carbon material such as graphite is used as a negative electrode, and a porous separator is used between the positive electrode and the negative electrode, such as a lithium-ion secondary battery, was first manufactured as a product in 1991. Since then, having higher energy density, the nonaqueous secondary battery has become rapidly widespread as a battery for a mobile instrument such as a mobile telephone which has been reducing in size and weight.
In addition, a lithium-ion secondary battery (large-capacity battery) having enlarged capacity to store power generated has been studied. As the large-capacity battery, there has been announced a battery manufactured by merely upsizing a conventional battery for the use in a mobile instrument.
In the lithium-ion secondary battery, an organic electrolytic solution is used as an electrolyte. Some measures are therefore taken to the battery to prevent incidents such as rupture and catching fire even under harsh use conditions. One of the measures is a “shutdown function”, that is, when the battery temperature rises, a separator melts to close holes of the separator to interrupt the current.
However, even a lithium-ion secondary battery to which such measures are taken experiences some problems associated with the safety of the battery. For example, when short circuit is caused by an external obstacle (such as penetration of a nail) or short circuit is caused by an internal obstacle (such as contamination by a foreign matter), the current flow is concentrated in the short-circuited part. The concentrated current causes resistance heating in the battery. The heating causes a chemical reaction of an active material or the electrolytic solution in the battery. As a result, so-called “thermal runaway” occurs in the battery. In the worst case, the battery comes to be ruptured or catch fire.
Japanese Unexamined Patent Publication No. HEI 11 (1999)-16566 proposes a solution for the above-described problem. Specifically, the publication proposes use of a separator in which a surface facing a positive electrode is coated with a metal having excellent oxygen absorption capacity selected from Ti, Al, Sn, Bi, Cu, Si, Ga, W, Zr, B and Mo; and/or a intermetallic compound formed by combining two or more of these metals; and/or an oxide of these metals.